Weight reduction, but retaining 50:50 balance

[Juriën]

I guess it would be quite easy to mess up the handling by removing the wrong parts and spoiling the weight distribution.

Well, it depends on what parts you remove really. For example it wouldn\'t matter that much if you removed the left door panel, and left the right panel (awkward choice of words), cause the weight difference can be ignored. It\'s when the difference is noticeable that it can be screwed up.

We also need to remember that the lower the centre of gravity the better too.

very true :yes That\'s why the BMW M3 CLS has a carbon roof.

Beware of Freddy! Rumour has it that he\'s got a monster on the way.  :burnout

What exactly are your plans TBlair, or is that a secret too?

[TonyBlair]

No secret mate. My plans for my car over the next 12 months or so is to achieve around 350~ rwbhp - on stock porting and using my existing Hitachi H12\'s (couldn\'t really say "stock twins" as they\'ve been uprated to 360 degrees thrust bearings).

I appreciate that there is a limit to what you can produce from a RX7 running stock porting and stock\'ish twins. Which is why I\'ve become very interested in the idea of weight reduction and I\'ve been trying to think about how much I can realistically lose (in terms of weight) out of the car without messing up the handling, or reducing the ride quality e.g. removing sound proofing etc.

I think the standard RX7 weighs 1270kg, so what do you guys reckon is achievable? 1200kg? 1150kg? I can only imagine what 350rwbhp feels like on a car that weighs 1150kg - acceleration should be neck-breakingly fast, and with a top-end of around 160~?

Cheers,
Jon

[iaint]

When you\'re talking cornering and car weight it\'s very simple - lighter is better.

The actual physics is hugely complex and we\'re talking about a car with just mechanical grip (not from downforce - although a GT wing could add at the rear preventing high speed oversteer).  Look at the effort (cost) involved in making F1 and GT cars go round bends quickly!

Simply speaking the grip added by extra wieght (tyre forced against tarmac) is outweighed by the centripetal forces generated when cornering.

i.e. the grip added by an extra 50kg is much less than is needed to counteract the extra outward force of the 50kg.  Same goes for wiehg over back wheels in a drag car, it\'s better to have less inertia to overcome than a tiny amount more grip.

Also, the way tyres work means that doubling the wieght on a tyre does not double the grip.  Doubling the wieght does double the forces acting on the car though (inertia and momentum).

IIRC the relevant maths is:

F = Ma

or

Force = Mass x acceleration.

For a given acceleration (more complex iwhen cornering because it\'s angular accelleration but for a given poing in a corner this equation works) the amount of Force is proportinal to Mass.

Alternatively - if the tyres can grip at a given Force and weight is constant you can calculate the acceleration that can be applied befoer traction is lost.

In reality the maths is way more complex as we\'re dealing with a very complex system (slip angles, rotation round the car\'s CoG, etc) but that\'s about as close as we need to go to realise that low weight is massively important for fast cornering (and acceleration!).

[TonyBlair]

Many thanks for your contribution Iaint!

All the best,
Jon

[nikp]

There\'s also the issue of sprung weight, and unsprung weight, and the ratio between them. I think sprung weight is everything that sits on top of the suspension, and unsprung weight is everything coming off the suspension (suspension arms, springs, dampers, wheels, etc) - this is a simplistic view of such items, as not every single part of the suspension components is included as unsprung...i think!

[TonyBlair]

I knew this thread was going to get complex   :eek

I\'m going to have a browse of the manual later to see if there are are quoted numbers for the weight of individual components. I think there\'s a whole section at the back of the book dedicated to the specifications of the car. If I find anything I\'ll post up the information. I think that\'s about all I can manage to contribute to this thread since a lot of this technical information about physics etc. is going straight over my head  :eek

Cheers
Jon

[nikp]

When doing my clutch replacement, i found several parts that are quite heavy and can be removed. Here\'s a list:

Poper Plant Frame
Gearbox
Wheels
Driveshaft

Of course, you probably won\'t go anywhere after that, but your car will be a lot lighter! :p

Hope that helps!

[Mave]

The actual physics is hugely complex and we\'re talking about a car with just mechanical grip (not from downforce - although a GT wing could add at the rear preventing high speed oversteer).  Look at the effort (cost) involved in making F1 and GT cars go round bends quickly!!

Not to mention the drag...

Alternatively - if the tyres can grip at a given Force and weight is constant you can calculate the acceleration that can be applied befoer traction is lost.

Doesn\'t this essentially mean that, for a given friction coefficient, your maximum acceleration is independent of mass?

I think that one of the most overlooked things about mass is that as well as your tyres dragging your car aroud a corner, they are turning the car about it\'s own axis. Whilst the forces required to accelerate in a straight line or in a corner go up linearly with mass, the forces required to rotate the car go up with mass squared (assuming you keep the same mass distribution) so you end up using more of your available grip just to get the car turning

[TonyBlair]

Of course, you probably won\'t go anywhere after that, but your car will be a lot lighter! :p

May as well lose the engine while you\'re at it  

On the subject of these parts, I guess you could opt for; -

Poper Plant Frame - don\'t know what this is  :confused:
Gearbox - lightweight gearbox, is there such a thing?
Wheels - I\'ve heard magnesium alloy is a good choice, as it\'s light?
Driveshaft - carbon driveshaft perhaps?

Cheers
Jon

[nikp]

Power Plant Frame is a big long bit of metal (Kinda looks like a I beam used in building sky scrpaers!) that connects the gearbox to the diff - it both keeps the engine/gearbox assembly level and the diff level too. I believe it adds some rigidity to the whole car as well. As to your options on reducing the weight of this thing are, i don\'t know. Wouldn\'t wanna mess with it really, due to the structural necessity of it.

[nikp]

Magnesium alloy are nice \'n\' light - used in F1. Probably nice \'n\' expensive too!

Yup, carbon driveshaft should do it.

[TonyBlair]

I\'ve been trying to find out some information about the different types of metal used for wheels, so far the list is; steel alloy, aluminium alloy and magnesium alloy. There\'s probably a few more types out there but I\'ll be damned if I can find them!

I\'m guessing that steel is the heaviest, then aluminium, then magnesium - this I\'ve deduced by what they cost. I find it annoying that even reputable wheel manufacturers/suppliers do not list the weight of the indivdual wheels they sell - just the size it seems.

I suspect there\'s not much to be gained, or lost as the case may be, from using wheels made of different materials. However in the spirit of keeping this thread rich with information I\'d like to ask whether anyone has any links to other sites with technical information about wheel designs, in particular with reference to their weight.

Cheers
Jon

[TonyBlair]

Just read an interesting fact... "RENESIS is an improvement from the previous 13B-REW although 30% lighter, smaller, and naturally-aspirated." - extract from; //www.rotaryengineillustrated.com/renesis.php

So 30% weight loss on the block itself can be achieved from swapping the 13B-REW to a RENESIS, then then of course you lose the the turbos - swings and round-abouts I guess, but nowhere near as fun.

Cheers
Jon

[Mave]

I\'ve been trying to find out some information about the different types of metal used for wheels, so far the list is; steel alloy, aluminium alloy and magnesium alloy. There\'s probably a few more types out there but I\'ll be damned if I can find them!

I\'m guessing that steel is the heaviest, then aluminium, then magnesium - this I\'ve deduced by what they cost. I find it annoying that even reputable wheel manufacturers/suppliers do not list the weight of the indivdual wheels they sell - just the size it seems.

I suspect there\'s not much to be gained, or lost as the case may be, from using wheels made of different materials. However in the spirit of keeping this thread rich with information I\'d like to ask whether anyone has any links to other sites with technical information about wheel designs, in particular with reference to their weight.

Cheers
Jon

Generally you are correct with the ranking of weight, but.... some aftermarket aluminium of magnesium designs are heavier than steel rims! (although better looking , forged wheels will be stronger or lighter than cast, and whilst magnesium is lighter than aluminium it is a lot more susceptible to corrosion which is worth remembering if you are using it on the road.

[paultakata]

You can also get carbon fibre composite wheels, rather hard to find though.

This company has some for sale: http://www.autovelocityinc.com/wheels.htm and there is a British company called Dymag who make some as well: http://www.dymag.com/

Apparently an 18x10.5 Dymag magnesium/carbon composite wheel weighs 7.2 kilos, 15.89 lbs.

I wouldn\'t like to be driving at "private test facility" speeds when the carbon rim broomed due to an impact though! :eek